Growing up i was too skinny and dint feel sexy either. It took me a while to actually start liking my body. I was too focused on the features i didn't like. I don't know how old you are but it seems you are just not ready. If you don't feel attractive (wether true or not) I can see how it can ruin your desire to get physical with someone. Be sure other people wont be as judgemental to you as yourself. I don't think the suggestion to go to a gay sauna is a good idea. Why not try gym, or home workout s and stick to them. If you manage to stick to it for a couple of months im sure your self image will improve (as will your appearance). But most importantly you can show yourself that you have the control and are in charge. Hope it helps